Enhanced Catalytic Activity for Methane Combustion through in Situ Water Sorption

Although palladium-based materials are efficient catalysts for methane combustion, H2O-poisoning remains a significant problem at low operating temperatures (<450 degrees C). It is a significant research challenge to find catalysts that can withstand water while delivering rates comparable to the dry PdO phase. Therefore, an in situ water sorption strategy could be advantageous in maintaining high rates for the reaction. Here, we investigate water sorbents that are used to remove the water produced by the reaction, which significantly enhances the activity of a Pd/CeO2 catalyst for methane combustion. The results reveal that the addition of zeolites or alumina to the Pd catalyst can transiently reduce the water concentration in the reactor and thus lead to increased catalytic rates. However, due to the limited number of sorption sites, hydroxylation of the PdO phase takes place rapidly, causing the catalyst to quickly exhibit the same activity as without the sorbent after a transient state. However, by using a stronger sorbent such as CaO mixed with the catalyst, it is possible to obtain higher catalytic activity that is maintained for longer periods of time. This latter system exhibits a 6-fold improvement in transient conversion of methane compared to that of the control catalyst. Meanwhile, long-term stability tests show catalytic activity higher than that without sorbent for extended periods of time. Mechanistic analysis demonstrated that the number of adsorptive sites and the water sorption energy on the sorbents both influenced the water sorption efficiency. Although the sorbent needs to be regenerated, we envision this strategy being useful in conditions where regeneration can be performed without disruption of the catalytic performance. Overall, this work opens an avenue for promoting the methane combustion activity of catalysts by in situ water sorption.